This invention relates generally to the field of recycling solid waste materials, and more particularly to processes for recycling and reclamation of useful materials from rubber vehicle tires.
The disposal of tires from automobiles, tractors, trucks etc. is a serious problem in the modern automotive economy. While some tires are suitable for retreading, many are not or the quality of retreaded tires is not acceptable to many consumers. Therefore, the disposal or recycling of scarp tires becomes a major concern.
Many methods of disposal for scrap tires have been used. Obviously, the smoke and noxious chemicals generated by uncontrolled incineration of scrap tires are no longer acceptable. Various methods to incinerate tires while avoiding air pollution have been tried, including a method of high temperature vacuum pyrolysis.
Burial in landfills is becoming increasingly difficult as landfills are closed and acceptable sites for new landfills become harder to locate. Also, the problem of malicious or accidental fires involving large accumulations of scrap tires is a serous concern to communities, leading to increased pressures for an effective solution to the tire disposal problem. The problem of disposing scrap tires is becoming such a concern that legislation is under consideration requiring tire vendors to demonstrate acceptable disposal means for each new tire placed into the stream of commerce.
Faced with the difficulities of disposal, much consideration is naturally given to recycling scrap tires for the extraction of as much useful material as possible. Finely shredded rubber from scrap tires (often called "granulite" or "crumb rubber") has been used as a fuel source commonly known as "tire derived fuel". Such fuel has been used as an energy source in the generation of electricity, in the production of paper and cement, and in various other industries. Apparently, such burning of suitably fragmented tires can be performed in an environmentally save manner if proper precautions are taken.
Also, recycled rubber from tires is used as a component of various products commonly known as "tire derived products". Such products include asphalt paving mixtures and as extenders in a variety of rubber products such as roofing materials, walk pads, carpet and flooring underlay and other products. More such products are being demonstrated as time proceeds.
Tires typically consist of a reinforcing cord (often steel cord in modern tires), woven typically into a mat, and circumferentially surrounding the tire beneath the road-contacting surface of the tread. A primary purpose of this cord is to strengthen the tire against damage from puncture or impact with holes or road debris. Modern steel cording is typically a rather high quality steel.
Tires also typically contain several heavy metal bands or wires imbedded in the rubber for support on the inner circumference of the tire, where the tire contacts the rim of the wheel. This "bead" is typically a very strong and heavy steel, difficult to shred in many mechanical shredders or causing excessive wear on the shredding devices.
In addition to cord and bead, tires also typically consist of several layers of synthetic or naturally-occurring products arranged into tread, various other plies, sidewalls, inner lining, etc. The precise composition and configuration will typically vary considerably, depending on the type of tire, its intended use and service life, desired market price, and consumer preferences. Such "rubber-like" materials may have a variety of different chemical compositions, and be arranged in differing physical configurations for each tire type, manufacturer, etc. For convenience, we will use the term "rubber" to denote all such rubber-like materials comprising vehicle tires.
Additional support and cohesion is introduced into many tires by using a fabric cord (typically nylon or rayon) imbedded in the rubber and typically located in regions of the tire not contacting the road.
Thus, for purposes of this description, we identify four components of typical vehicle tires: 1) steel cords, 2) bead 3) fabric (or non-metallic) cords, 4) rubber.
Rubber and fabric cord can typically be burned as tire derived fuel, or used in other tire derived products (occasionally following further separation and/or processing). The steel cords and beading will typically have some value as scrap metal, if separated in a reasonably clean state. A method for effecting such a separation is the primary purpose of the present invention.
Mechanical shredding is the typical method for recycling in use today. However, mechanical shredding typically does not separate the steel cords (or other cording materials) from the shredded rubber tire. Therefore, the shredded rubber is rendered useless for many additional purposes. The economic value of the scrap steel is effectively lost. Following combustion of the rubber/steel, the surface of the steel is typically too encrusted with ash and combustion by-products to make separation and recovery economically worthwhile.
In addition, typical mechanical shredding operations produce particles of rubber scrap generally too large for convenient use as a fuel or as an additive in asphalt paving. Further processing, adding to the costs, would be required to derive economic benefit from reuse of such component materials (although the patent of B. H. Granite U.S. Pat. No. 4,015,782 addresses this problem).
Wear on mechanical shredding devices is a serious concern. For this reason larger tires, such as truck or tractor, are typically not recycled. Also, even for passenger car tires, the heavy steel bead can often produce unacceptable wear on shredders. It is often recommended that tires be "debeaded" before shredding (ie, physically cut and remove the bead and immediately surrounding rubber from the rest of the tire). Clearly, such debeading adds to the cost of tire recycling and leaves the bead and attached rubber to be disposed.
Efforts to ease such problems of shredding have included the use of cryogenic fluids (liquid nitrogen) to crystallize tires into brittle matter, allowing fragmentation by mechanical crushing (U.S. Pat. No. 4,813,614). The economic advantages of using liquid nitrogen to dispose of garbage have yet to be demonstrated.
The present invention describes a method for shredding scrap tires using high-pressure water in which the steel or other cording material is naturally separated from the shredded rubber, and the rubber is produced in sufficiently small sizes to be useful in other applications (asphalt, fuel source, etc.) without further processing. Also, as a non-contact process using highly pressurized water, the problem of mechanical wear on shredders is avoided, leading to the possibility of recycling many types of tires. Also, the bead of the tire presents no particular problem for the present invention, being stripped of rubber by high pressure water without difficulty. The great majority of steel (and all of the bead) is left behind as the rest of the tire is fragmented and washed into the effluent water stream. However, small "whiskers" of steel cord may be broken from the steel cord and find itself washed into the effluent stream. (This happens more often with damaged tires presented for recycling in which the steel cord is already fragmented of partially fragmented). Form many applications (ie as tire derived fuel) trace amounts of steel are not a concern. However, for some applications (ie asphalt additive) even minuet quantities of steel are unacceptable. In contrast to other shredding procedures, the present invention produces clean steel whiskers having little surface contamination even when washed into the effluent stream. Thus, magnetic separation of trace steel from the fragmented rubber is easily accomplished.
The present invention should be contrasted with that of Tugov (USSR Patent 213,339) in which thin streams of a temperature-controlled liquid are directed under pressure to a combination of polymer (rubber) and a fabric or thread cord. The resulting effluent liquid stream contains a mixture of fragmented polymer and fabric/thread, requiring further separation and processing. Thin streams of liquid under pressure are often used to cut materials (even steel or concrete), apparently accomplished also by Tugov. The control of temperature is apparently intended to avoid damage to the polymer or fabric. The present invention requires no temperature control and specifically avoids using thin streams of liquid precisely for the purpose of avoiding unwanted cutting of the cording materials (and the attendant need for significant separation/processing of the materials in the effluent stream).